This invention relates generally to cranes, and, more particularly, to an articulated boom crane with a main boom and an upper secondary boom with extensible members.
Presently available boom cranes are generally either of the straight boom type or the articulated boom type. A conventional straight boom crane includes a main boom with telescopically extensible members, and a boom tip assembly supported at the end of the farthest reaching extensible member. The boom tip assembly contains sheaves to support and guide a cable, which carries a load to be lifted by the crane. Typically, the extensible members are hydraulically controlled and may be retracted into the main boom, or projected therefrom as a longitudinal extension of the main boom.
A conventional articulated boom crane includes a main boom and a secondary boom, with the secondary boom being pivotally connected to the upper end of the main boom. The secondary boom often includes telescopically extensible members, which are hydraulically controlled and are retractable into the secondary boom and extensible therefrom as a longitudinal extension of the secondary boom. As in straight boom cranes, a boom tip assembly is supported at the end of the farthest reaching extensible member. Such articulated boom cranes are shown in my U.S. Pat. Nos. 3,754,666 and 3,884,359.
Articulated boom cranes are extremely versatile, and can handle loads in a number of positions in which lifting would be difficult, if not impossible, for a straight boom crane. However, there are certain situations in which, because of the clearance requirements for unfolding and operation of the main and secondary booms of an articulated crane, a straight boom crane is required. For example, a straight boom crane is needed to lift a load positioned close to the crane in a structure providing low overhead clearance, or when the crane must be positioned close to a structure and it is necessary to reach with a load up and between closely spaced trusses or pipes.
A disadvantage of articulated boom cranes of the prior art has been their inability to operate in such situations in the same manner as a straight boom crane, with the telescopically extensible members serving as longitudinally adjustable extensions of the crane's main boom. Thus, in many instances where the aforementioned situations are encountered on a job, the otherwise preferred articulated crane cannot be used, or both an articulated and a straight boom crane must be kept at the job site in anticipation of the need for a straight boom crane.
It will therefore be appreciated that there has long been a need for an articulated boom crane that can be conveniently converted for straight boom crane operation. The present invention fulfills this need.